The invention relates to a circuif for increasing the impedance of a winding which is wound around two cores of soft magnetic material.
In communications and measurement technology, use is frequently made of autotransformers and isolating transformers operative for stepping down or stepping up an alternating voltage, with it being desired that the step-down or step-up ratio be determined as exactly as possible by the winding ratio of the transformer. To meet this requirement, it is necessary to use transformers having a very high principal impedance compared to the ohmic winding resistances and stray impedances. This high principal impedance is also necessary in order to keep low the errors attributable to loading of the AC voltage source or to resistive or other voltage drops on the supply lines.
Additionally, there is considerable interest in inductors of high inductance and relatively small size, and also in inductors whose inductance can be varied by electronic means over several order of magnitude (powers of ten).
For transformers which are to serve as very high-precision inductive voltage dividers or matching transformers, the ratio of principal impedance to the winding resistances and stray impedances which can be achieved using conventional soft magnetic core materials is too low. Furthermore, due to the low principal impedance, the magnetizing current of the transformer loads the AC voltage source connected to the transformer and produces undesired voltage drops on the supply lines. This is particularly the case in the low-frequency range.
In general, inductors of very high inductance can be made using known core materials only by employing a large number of winding turns. However, that expedient results in a high ohmic winding resistance and poor quality of the finished inductor.
In the aforementioned cases, improvements can be achieved only by resort to electronic means. Gibbings (D. L. H. Gibbings, "Circuit for Reducing the Exciting Current of Inductive Devices," Proc. IEE. vol. 108 B, 1961, pp. 339-343) has disclosed a circuit for increasing the impedance and reducing the magnetizing current of a winding which is wound around two cores of soft magnetic material. With this circuit, there is provided on one of the cores an indicator winding the voltage across which is amplified by an electronic amplifier and then applied to a magnetizing winding on the other of the cores. In this way, Gibbings causes a great part of the magnetic flux to be established by the electronic amplifier, with only a relatively small magnetizing current flowing in the winding around the two cores and serving to establish a magnetic flux in the core around which the indicator winding is wound. The reduction in the magnetizing current achieved with this circuit is limited particularly at low and high frequencies, since at such frequencies one is in effect dealing with a two-transformer feedback circuit in which the gain of the amplifier, to stabilize against self-excitation, must not be selected too high and furthermore must drop off in direction towards low frequencies and also in direction towards higher frequencies.